1. Field of the Invention
The present invention relates to a manhole cover type omnidirectional antenna, and more particularly, to a manhole cover type omnidirectional antenna installed in a manhole cover of a manhole horizontally disposed to correspond to the Earth's surface to remotely collect and manage various types of sensing information under the ground and configured as a wireless sensor network or a wireless wide area network for communicating with a gateway above the ground.
2. Discussion of Related Art
Generally, a manhole cover installed on the Earth's surface is installed in a metal medium such as iron, zinc or the like, and the manhole cover needs to have a structure which does not protrude from the Earth's surface to prevent damage, performance degradation, etc. due to external environment.
Types of antennas applicable to the manhole cover include a patch antenna in a planar type structure, a small-sized dielectric antenna, and the like.
In addition, as a technology that establishes a system by applying such a common antenna, for example, there is a technology disclosed in United States Patent Laid-Open Publication No. US20010011009 entitled “Underground Information Communication System and Related Manhole Cover.”
However, a plurality of manhole cover antennas installed at arbitrary locations need to be able to wirelessly communicate with a gateway on the ground. A manhole cover antenna requires omnidirectional characteristics from a horizontal plane. Here, long-distance communication efficiency can be relatively increased as an angle formed by a main beam direction from a vertical plane and the Earth's surface is decreased.
In the case when an antenna main body is installed inside a manhole apparatus without having a portion protruding from an upper surface of the manhole, the manhole apparatus manufactured of a metal influences designed radiation characteristics and radiation gain of the antenna main body. As a result, realizing a high performance antenna while having a small radiation angle with respect to the Earth's surface becomes very difficult.
FIG. 1 shows availability of communication depending on a difference in a main beam direction according to a conventional technology.
Referring to FIG. 1, there are a first sensor node 10 and a second sensor node 20 in an underground space U at a lower level than the Earth's surface S. The first sensor node 10 and the second sensor node 20 are electrically and respectively connected to internal antennas 13 and 23 installed in manhole covers 11 and 21.
A gateway 30 and a gateway antenna 31 are installed at a predetermined location in a region in which a plurality of manhole covers 11 and 21 are located to communicate with the first sensor node 10 or the second sensor node 20. Particularly, the gateway antenna 31 is located at a predetermined height h from the Earth's surface S.
In the case in which the internal antenna 13 or 23 for the first sensor node 10 or the second sensor node 20 is installed in the manhole cover 11 or 21, the internal antenna 13 or 23 is at an equivalent level with the Earth's surface S. When the height h of the gateway 30 or gateway antenna 31 located is considered, the internal antennas 13 and 23 cannot have omnidirectional characteristics.
Radiation 22 performed by the internal antenna 23 of the manhole cover 21 connected to the second sensor node 20 is formed along a direction perpendicular to the Earth's surface S (for example, a right angle).
As a comparative example, radiation 12 performed by the internal antenna 13 of the manhole cover 11 connected to the first sensor node 10 can be formed along a direction corresponding to an inclination angle relatively smaller than the right angle with respect to the Earth's surface S.
Here, although distances g from the gateway 30) to the first sensor node 10 and to the second sensor node 20 are the same, actual communication distances L1 and L2 can be different depending on directions and angles of the radiations 12 and 22.
Meanwhile, as a conventional technology, the most typical antenna of an omnidirectional antenna is a monopole antenna. Generally, the monopole antenna is installed perpendicular to the Earth's surface. Therefore, the monopole antenna has difficulty in being operated inside a manhole cover formed of a metal.
On the other hand, a patch antenna, a planar antenna, and a small-sized dielectric antenna can be easily installed in a manhole cover.
However, when such a patch antenna, a planar antenna, or a small-sized dielectric antenna is installed inside a manhole cover or inside a manhole, difficulties can be faced due to not obtaining omnidirectional characteristics therefrom. Accordingly, development of an antenna having a structure by which radiation characteristics of the antenna is improved while being easily applicable to a manhole cover is urgently required.